It is rare that a single material will have all (or even several) of the typically desired characteristics for a particular application. A classic example is that strong materials are typically heavy. Another example is that the best thermal insulators (e.g., solid aerogels) are typically rigid, fragile, and have attendant low density. Such issues lead to classical “design trade-offs” and encourage the development of new materials at the microscopic level. Alternately, materials can be processed or treated at the macroscopic level to enhance desirable properties while attempting to minimize the undesirable “side effects” of the process or treatment. Although the process or treatment is at the macroscopic level, the resultant changes are typically at the microscopic level. A classic example is heat treating metals to increase hardness, usually at the expense of increased brittleness.
Thus, it is well known that one specific material does not typically possess all of the desired and/or required characteristics for a particular application. For example, high strength is often at the expense of high weight, high density, high rigidity and incompressibility. Another example is thermal insulation, which has low thermal conductivity, typically at the expense of compressibility; which, in turn, lowers its effective thermal resistance. Composite materials attempt to alleviate this by combining materials with desirable properties for a particular application. There usually remain, however, undesirable “side-effects” (design trade-offs). To illustrate further, a comparison is made between two types of thermal insulation, namely: foamed neoprene and syntactic foam.
Foamed neoprene is currently used for underwater diver thermal insulation. It is a good insulator because it contains small “pockets” of gas trapped in closed internal cells. Since gas has a low thermal conductivity, foamed neoprene is a good insulator as long as the closed internal cells retain their integrity, including their volume of gas. However, as a diver goes to increasing depth in water, the associated and inevitable increase in local hydrostatic pressure compresses the gas trapped in the closed internal cells. This reduces their volume and the foamed neoprene is said to “go flat.” Since the thermal resistance is the thickness divided by the thermal conductivity, this causes dual disadvantages inasmuch as its thickness is reduced and its effective thermal conductivity is increased. That is to say, its insulating capability is reduced (often unacceptably) by two mechanisms.
Syntactic foam is a composite material composed of a matrix material and a filler material. One use of syntactic foam is for thermal insulation in high-pressure environments, such as thermally insulating deep-ocean oil pipelines. For this application, the matrix material is typically plastic and the filler material is typically hollow micro- and/or macro-spheres. The hollow micro- and/or macro-spheres may be gas-filled or evacuated. The matrix material serves to hold-in-place the relatively low thermal conductivity hollow micro- and/or macro-sphere filler material. As the volume fraction of the filler material increases, the effective thermal conductivity of the syntactic foam decreases, i.e., the effectiveness of the insulation increases. This is done, however, at the expense of increased stiffness. This increased stiffness is a distinct disadvantage of syntactic foam in that it does not conform well to contours, unless it is molded-in-place. Furthermore, as the volume fraction of lower conductivity inclusions is increased, to increase its insulation capability, current syntactic foam suffers from a decrease in flexibility. In other words, the better the insulation, the stiffer it becomes. This makes it unsuitable for insulation when flexibility is required—such as for: insulating contours, clothing and underwater diver thermal protection. The differences in thermal resistance between foamed neoprene and syntactic foam are even more striking. They would amount to about a six-fold advantage of the syntactic foam at depth of 350 feet of sea water. Currently, the stiffness of syntactic foam makes it impractical for use in garments such as dive suits.
The present invention is directed to overcoming these and other deficiencies in the art.